server: refine the connection counter logic (#54546)

[ti-chi-bot]

This is an automated cherry-pick of #54546

What problem does this PR solve?

Issue Number: close #54545, close #54428

Problem Summary:

This PR solve two problems:

1. The inc/dec of the connection doesn't organize in pair, so the connection count may decrease below zero. It actually increases in registerConn, and decreases in conn.Close() (if the conn.getCtx() is not nil). If the connection exists before handshake successfully, for example authentication failed, the total connection count will decrease one 🤦.
2. SET SESSION_STATES (usually used by tiproxy) didn't INC/DEC the connection count for the resource group if it's changed.

What changed and how does it work?

This PR reorganized the codes INC/DEC the metric gauge in the following pattern:

1. It's increased in newConn with default resource group name, so that it can make sure the connection count will always increase.
2. It's decreased in closeConn. If the connection doesn't have a valid context, it'll decrease in the default resource group.

Other use case of the ConnGauge always comes in pair: increase one and decrease another one. This PR provides a method SetResourceGroupName to change the session vars and always INC/DEC the metric counter. It'll be called in SET SESSION_STATES, SET RESOURCE GROUP and authentication (because the user may carry a resource group info).

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Release note

Fix the issue that the connection count is not correct if some connection exits before handshake finishes.
Fix the issue that with tiproxy and non-default resource group, the connection count of each resource group is not correct.

[YangKeao]

/retest

[YangKeao]

LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-30 09:05:30.859552639 +0000 UTC m=+260247.139600709: ☑️ agreed by xhebox.
• 2024-08-01 06:39:12.522267174 +0000 UTC m=+424268.802315242: ☑️ agreed by YangKeao.

[easonn7]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: easonn7, xhebox, YangKeao

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Needs approval from an approver in each of these files:

• OWNERS [YangKeao,xhebox]
• pkg/sessionctx/variable/OWNERS [easonn7]

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[codecov]

Codecov Report

Attention: Patch coverage is 98.96907% with 1 line in your changes missing coverage. Please review.

Please upload report for BASE (release-8.1@8737e7d). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.1     #54612   +/-   ##
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  Coverage               ?   71.1603%           
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  Files                  ?       1465           
  Lines                  ?     422446           
  Branches               ?          0           
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  Hits                   ?     300614           
  Misses                 ?     101323           
  Partials               ?      20509           

Flag	Coverage Δ
unit	71.1603% <98.9690%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	53.9957% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	41.3131% <0.0000%> (?)

[YangKeao]

/retest

[YangKeao]

/retest

[xhebox]

/retest